It is known in the art to use a spinning inertial mass called a "flywheel" as an energy storage/retrieval system. It is also known to dump energy into the flywheel using electromagnetic force to "spin-up" or accelerate the flywheel to a predetermined rotational speed, such that it "stores" kinetic energy. The higher the speed, the more energy the flywheel stores. To spin-up the flywheel, the system acts in a motor type operation with the flywheel being the rotor. This stored kinetic energy can then be extracted from the spinning flywheel by use in a generator-type action using electromagnetic fields to "spin-down" or reduce the rotational speed of the flywheel.
When no energy is being input into the flywheel nor extracted from the flywheel, the flywheel is said to be "freewheeling" or in a "freewheeling" mode. During freewheeling, it is desirable to minimize the electromagnetic losses on the flywheel due to the motor/generator configuration, thereby avoiding extracting wasteful energy from the spinning flywheel. If such losses are not minimized, they cause the flywheel rotational speed to be reduced, thereby reducing the amount of energy available to be extracted.
Numerous configurations exist in the art for this motor/generator flywheel system. For example, permanent magnets may be located on the spinning rotor portion of the flywheel and motor coils may be located on a stationary or stator portion of the system. In that case, when the rotor is freewheeling, the coils are typically open-circuited through some maximum impedance to minimize the current and to minimize the amount of electromagnetic force generated in the coils, thereby minimizing the amount of energy extracted from the rotor during freewheeling. However, even though the coils are open-circuited, eddy currents are still generated due to the rotating permanent magnets of the flywheel, thereby generating some magnetic fields which extract energy from the flywheel and reduce its speed.
Thus, it would be desirable to design a flywheel motor/generator system that minimizes electromagnetic drag on the flywheel during freewheeling.